Modern electronic circuits rely heavily on frequency sources for many applications. For example, radio frequency (RF) signals may be generated from a reference frequency source to convert a baseband signal to and from the higher frequency used to transmit the signal in wireless communication systems. Digital electronic circuits also depend upon frequency sources to generate a stable clock signal to control the operation of synchronous elements such as logic gates.
A notable characteristic of oscillating frequency sources such as crystal oscillators is the potential for unwanted leakage of harmonic energy into other circuit elements, creating spurious signals or “spurs.” The problems posed by these aggressor signals are exacerbated by trends in circuit design to integrate multiple analog and digital functions into a single chip, such as in system-on-a-chip (SoC) applications. The presence of unwanted spurs may result in a performance degradation of the wireless communications system. For example, spurs that couple into the receive chain of a wireless transceiver may be manifest as interference that reduces sensitivity and inhibits the ability to properly process the signal. Similarly, spurs coupling into the transmit chain may be inadvertently transmitted, resulting in interference with other devices. In light of these deleterious effects, minimization or mitigation of spurs is an important design consideration.
Different techniques for addressing problems associated with harmonic spurs exist, but typically involve undesirable compromises. For example, attempts may be made to filter a received signal at frequencies known to contain unwanted harmonic components. However, such techniques may also attenuate desired components of the received signal. Other techniques may include attempts to reduce the amplitude of a spur by injecting a cancelling tone, which may require additional circuitry and power consumption.
In particular, spurs that appear at the input of the low noise amplifier (LNA) of a receive chain may be especially problematic. To help reduce the degradation that results from such spurs, the LNA may be designed with a differential topology, requiring additional circuit area and power as compared to a single-ended topology. Further, transceiver architectures and operational frequencies that would otherwise be advantageous may be avoided in an attempt to minimize the undesirable effects of spurs.
Accordingly, there is a need for systems and methods of providing a frequency source that minimizes the impact of harmonic spurs. Likewise, there is a need for a frequency source that imposes fewer constraints on the design of a wireless communications device. This invention satisfies these and other needs.